In applications where a screw is employed to secure workpieces together, the screw may be inserted without first preparing the workpiece, or a pilot hole may be drilled. One reason for drilling a pilot hole is to reduce the turning torque required to drive the screw into the material. The turning torque of a screw is that force required to cut a channel or groove in the material into which the screw is driven, and to overcome the sliding friction between the material and the thread and shaft of the screw as it is turned. The turning torque is of particular concern when driving long, large diameter screws into hard materials. In order to reduce friction, a lubricant may also be applied to the thread of the screw. However, even if the material and the screw are prepared as described above, the torque required to drive the screw completely into the material may be too large to permit the comfortable use of a hand-held screwdriver.
Conventional screws comprise a metallic shaft having an enlarged head on a first end, a tip on a second end and a uniform helical thread running from the tip toward the head. The thread at the tip cuts a helical channel into the workpiece as the screw is rotated by a turning tool. The trailing surface of the thread determines the holding power of the screw which can also be described as the resistance to screw pull-out from the workpiece. Although screws having a shaft length on the order of two inches and longer are typically used to fasten two workpieces together, the thread of such screws has been given little attention.
There are few references pertaining to thread shape. U.S. Pat. No. 5,226,766, which issued on Jul. 13, 1993 and is entitled "Bone Screw With Improved Threads", discloses a bone screw having a helical thread that increases in thickness from the tip to the head of the screw. Although such a screw will exhibit increased resistance to pull-out when fully inserted, the turning torque required to drive the screw into a material increases with screw length. Thus, the frequently-encountered problem of overcoming the turning torque when driving a long screw into a workpiece encountered by, for example, home builders and wood hobbyists, has not been addressed.